Nature of the $A$ Phase in ${\mathrm{CeCu}}_2{\mathrm{Si}}_2$

Neutron diffraction experiments have been performed on a magnetically ordered ${\mathrm{CeCu}}_2{\mathrm{Si}}_2$ single crystal exhibiting $A$-phase anomalies in specific heat and thermal expansion. Below $T_{\mathrm{N}}\approx 0.8\mathrm{K}$ antiferromagnetic superstructure peaks have been detected. The propagation vector of the magnetic order appears to be determined by the topology of the Fermi surface of heavy quasiparticles as indicated by renormalized band-structure calculations. The observation of long-range incommensurate antiferromagnetic order as the nature of the $A$ phase in ${\mathrm{CeCu}}_2{\mathrm{Si}}_2$ suggests that a spin-density-wave instability is the origin of the quantum critical point in ${\mathrm{CeCu}}_2{\mathrm{Si}}_2$.

Figure 1

(a) Specific heat plotted as $C/T$ versus temperature $T$ of the ${\mathrm{CeCu}}_2{\mathrm{Si}}_2$ single crystal in zero magnetic field and $B=2\mathrm{T}$ applied along the $c$ axis. (b) Temperature dependence of the thermal expansion coefficient $\alpha$ along the $a$ axis in ${\mathrm{CeCu}}_2{\mathrm{Si}}_2$. The inset shows the hysteretic behavior of $\alpha$ at low temperatures.

Figure 2

(a) Intensity map of the reciprocal $(hhl)$ plane around $q=(0.210.211.45)$ in ${\mathrm{CeCu}}_2{\mathrm{Si}}_2$ at $T=50\mathrm{mK}$ and 1 K. (b) $\omega$ scan across the magnetic peak at $q=(0.2150.2151.47)$ at 50 mK and 1 K [data shown in (a) and (b) taken on E6]. (c) $\omega$ scan across the magnetic peak at roughly the same wave vector $q$ as in (b) at 0.5 K and 1 K for the second crystal measured on E4 (“res” denotes the instrumental resolution).

Figure 3

(a) Comparison of the measured propagation vector $\tau$ with theoretical intensity map for the wave vector dependent magnetic susceptibility ${\chi }_0(q)$ of noninteracting quasiparticles in the reciprocal $(hhl)$ plane of ${\mathrm{CeCu}}_2{\mathrm{Si}}_2$. The calculations are performed at $T=100\mathrm{mK}$ using the following parameters to characterize the low-energy excitations: $T^{*}\sim 10\mathrm{K}$, ${\Delta }_{\mathrm{CEF}}=330\mathrm{K}$. The contribution of the incoherent background has been subtracted. (b) Fermi surface of the heavy quasiparticles in ${\mathrm{CeCu}}_2{\mathrm{Si}}_2$ as calculated with the renormalized band method. The vector $\tau$ connects parallel flat parts (“nesting”) of the Fermi surface.

Figure 4

Temperature dependence of the components of the propagation vector $\tau =(hhl)$ and of the intensity of the magnetic $(hh2\mathrm{\text{−}}l)$ Bragg peak in ${\mathrm{CeCu}}_2{\mathrm{Si}}_2$ (measured on E6). Dashed lines are only guides to the eye.